Tau Sagittarii

With an apparent visual magnitude of +3.3,[2] this is one of the brighter members of the constellation. The distance of this star from Earth is roughly 122 light-years (37 parsecs), based upon parallax measurements.[1]

This is a spectral type K1 giant star with about 1.25 M_☉. The stellar envelope is slightly cooler than the Sun with an effective temperature of 4,459 K, giving the star a light orange color. The interferometry-measured angular diameter of this star, after correcting for limb darkening, is 3.93 ± 0.04 mas,[9] which, at its estimated distance, equates to a physical radius of about 16 times the radius of the Sun.[10]

τ Sagittarii is a suspected double star although no companion has been confirmed yet. A lower metal content (Fe to H ratio is 54% lower than the sun's) and a high peculiar velocity (64 km/s, four times the local average) relative to the Sun suggest the star is a visitor from a different part of the Galaxy.

τ  Sagittarii is a red clump giant, a star with a similar mass to the sun which has exhausted its core hydrogen, passed through the red giant branch, and started helium fusion in its core.[11]

τ Sagittarii is the closest constellational star (a star that is part of the traditional outline of a constellation) to the origin of the 1977 Wow! signal.[12]

• The star forms part of simple asterisms:
  • γ Sgr, τ Sgr, δ Sgr, ε Sgr, ζ Sgr, λ Sgr, σ Sgr and φ Sgr — the Teapot.[13]
  • φ Sgr, τ Sgr, ζ Sgr, χ Sgr (double) and σ Sgr — the Returning Ostriches; in Arabic Al Naʽām al Ṣādirah often transliterated as Namalsadirah (النعم السادرة).[14]
    • Originally four stars in a numerical order: φ Sgr, τ Sgr, χ¹ Sgr and χ² Sgr[15]
  • ν Sgr, τ Sgr, ψ Sgr, ω Sgr, 60 Sgr and ζ Sgr, Al Udḥiyy — the Ostrich's Nest.[14]
  • In the entirely separate Chinese tradition — 斗 (Dǒu), meaning Dipper: τ Sgr, φ Sgr, λ Sgr, μ Sgr, σ Sgr and ζ Sgr. The star itself is 斗宿五 (Dǒu Sù wu, English: the Dipper's fifth star).[16]